In order to increase the sensitivity of a semiconductor device equipped with a photodiode having a p-n junction, the junction capacitance existing on the junction face of the photodiode must be reduced. Various means have been employed to this end.
First, the relation between the junction capacitance resulting from the photodiode structure and sensitivity will be explained, taking as an example a luminous energy detection circuit in charge-storage light
a sensor in which a photodiode is used as a light-receiving part. The conventional luminous energy detection circuits in charge-storage light sensors can be roughly divided into two types. One type has circuitry as shown in FIG. 4(a). These circuits store a charge for a certain period of time and detect the charge amount as a voltage. Circuits of this type are used in solid-image MOS sensors. The other type has circuitry a shown in FIG. 5(a). These circuits measure the amount of time before a certain voltage is reached.
The former type of circuit, shown in FIG. 4(a), operates according to the timing chart shown in FIG. 4(b). First, a transistor 16 is closed and, up until time t.sub.1, a junction capacitance in a photodiode 15 is applied with a reverse bias, and a charge is stored. Next, when transistor 16 is opened at time t.sub.1 and brought back to a closed state at time t.sub.2, a charge is generated in photodiode 15 corresponding to the light energy received during time T=t2-t.sub.1, and that much of the charge stored in junction capacitance 18 in photodiode 15 is discharged.
When transistor 16 is closed at time t.sub.2, a current flows through a load resistance 17 in order to charge the junction capacitance 18 in photodiode 15. An output voltage V shows at this time a maximum value Vo, as shown in FIG. 4(b). In this case, the maximum value Vo is presented in the following equation: EQU Vo=i.multidot.T/(C+C.sub.1 .sub.2) (1)
where i is a photoelectric current corresponding to light energy, C.sub.1 is junction capacitance 18 in photodiode 15, and C.sub.2 is parasitic capacitance 19 in the circuit.
Next the circuit of the latter type, shown in FIG. 5(a), operates according to the timing chart shown in FIG. 5(b). In this circuit, a comparator output V.sub.2 reverses when a potential V is equivalent to a reference voltage V.sub.ref in a comparator 20. Measuring this time t.sub.2 gives T=t.sub.2 -t.sub.1. In this case, T is expressed by the following equation: EQU T=(C.sub.1 +C.sub.2).multidot.V.sub.ref /1 (2)
Thus, by measuring Vo in the former case, and T in the latter case, the photoelectric current i corresponding to the light energy can be obtained, and light energy can be detected. The sensitivity of such a light sensor can be raised by elevating the V.sub.o relative to a given light energy in equation (1), and by shortening the T relative to a given light energy in equation (2).
In both cases, the photoelectric current i relative to a given light energy may be increased, and C.sub.1 +C.sub.2 may be decreased. However, there exists a lower limit to the parasitic capacitance C.sub.2, which is determined by the detection circuit, and no further reduction beyond this limit is possible. In order to increase the photoelectric current i, the expansion of the light receiving area is the most effective means, but an increase in the light-receiving area conversely reduces the resolution of the light sensor, and is hence undesirable. Therefore, to increase the sensitivity of the light sensor, junction capacitance C in the photodiode must be reduced.
The conventional structure of a photodiode with a p-n junction will now be explained with reference to FIGS. 3 (a) (b) and (c). FIG. 3(a) is a diagram view, and FIG. 3(b) is a cross-section of FIG. 3(a) sectioned along the A--A, line. When using an n-type silicon substrate as a low concentration layer 9 of the first conduction type, for example, an impurity, such as P (phosphorus) is diffused on a non-active region on the surface of the substrate to form a region 8 of the first conduction type, being an n-type high concentration semiconductor to isolate each sensor in which the active regions are used as isolation islands. Furthermore, B (boron) is diffused as an impurity over the active regions to form a region 13 of the second conduction type, being a p-type semiconductor. FIG. 3(c) is an expansion of region 13 of the second conduction type in FIG. 3(b). Region 8 of the first conduction type with high concentration and layers 9 of the first conduction type with low concentration surround the regions 13 of the second conduction type which constitute each sensor. In this case, while regions 8 and 9 are both first conduction type regions, region 8 of the first conduction type is normally set at a high concentration to isolate the sensors, and the layer of the first conduction type is set at a low concentration.
When a photodiode is applied with a reverse bias, depletion regions 14 are formed around the junction faces. In conventional photodiodes, regions 13 of the second conduction type occupy the entirety of the light receiving parts, except for the regions used for isolation, whereas the photodiode utilizes a p-n junction among region 13 of the second conduction type and regions 8 and 9 of the first conduction type.
Therefore, in conventional photodiodes, the junction capacitance C.sub.1 is proportional to this p-n junction area and interrelated with the area of light-receiving part.
As a means to reduce C.sub.1, another structure has been proposed as shown in FIG. 6. This structure forms a region 21 of the second conduction type in a ring-shape to reduce the p-n junction area and hence to reduce C.sub.1.
However, this structure has dimensional limitations, rendering it extremely difficult to make the width d less than 1 .mu.m. Also, to form region 21 of the second conduction type, a masking process and a process to introduce impurities are required, as in the conventional structure shown in FIG. 3.
An increase in the sensitivity of a light sensor requires the reduction of junction capacitance C.sub.1 in a photodiode. However, in conventional photodiodes, a reduction of the junction area to decrease the junction capacitance C.sub.1 results in a reduced light-receiving area, which ultimately reduces the photoelectric current i. Thus no improvement in the sensitivity of a light sensor can be expected.
Arranging the regions of the second conduction type in a ring-shape also produces a limited reduction of the p-n junction area, and the number of associated manufacturing processes could be more than if a ring shape is not used, but can never be less.